Blood clotting or hemostasis is an important protective mechanism of the body for sealing wounds caused from injury to the body. Hemostasis takes place in two phases. Primary (cellular) hemostasis serves to quickly stop bleeding and minimize blood loss. Primary hemostasis involves injured cells of the endothelium and the underlying layer of cells emitting signals that enable blood platelets (thrombocytes) to accumulate in a region of an injured blood vessel, forming a plug that provisionally seals the wound. Secondary (plasmatic) hemostasis or coagulation is initiated at the same time as primary hemostasis and involves a process by which blood clots. More specifically, coagulation is controlled by a signaling coagulation cascade consisting of thirteen coagulation factors that interact and activate each other. At the end of the coagulation cascade, fibrinogen is converted into fibrin. A network of fibrin fibers reinforces wound closure, and platelets and other blood cells get caught in this network and form a blood clot (thrombus). Lastly, platelets and the endothelium release growth factors that control a wound-healing process. At the end of these processes, the fibrin network is dissolved by enzymes in the blood plasma.
Hemostasis requires a subtle balance of procoagulants and anticoagulants such that circulating blood remains a relatively low-viscosity fluid and coagulation only begins in order to seal wounds. Procoagulants prevent excessive bleeding by blocking blood flow from a wound or damaged vessel, whereas anticoagulants prevent clots from forming in the circulating system, which could otherwise block blood vessels and lead to myocardial infarction or stroke.
The coagulation cascade of secondary hemostasis is based on catalytic conversion of fibrinogen, a soluble plasma protein, to insoluble fibrin. The enzyme catalyzing this reaction is thrombin, which does not permanently circulate in the blood in an active form but exists as prothrombin, the inactive precursor of thrombin. The coagulation cascade leading to active thrombin consists of two pathways, the extrinsic and the intrinsic pathways, which converge into a common pathway that includes active thrombin catalyzing the conversion of fibrinogen to fibrin. The extrinsic pathway is initiated at the site of injury in response to the release of tissue factor (factor III) and thus, is also known as the tissue factor pathway. Tissue factor is a cofactor in the factor VIIa-catalyzed activation of factor X (inactive) to factor Xa (active). The second, more complex, intrinsic pathway is activated by clotting factors VIII, IX, X, XI, and XII associated with platelets. Also required are the proteins prekallikrein (PK) and high-molecular-weight kininogen (HK or HMWK), as well as calcium ions and phospholipids secreted from platelets. Each of these constituents leads to the conversion of factor X to factor Xa. The common point in both pathways is the activation of factor X to factor Xa. Factor Xa is an enzyme (e.g., a serine endopeptidase) that cleaves prothrombin in two places (an arg-thr and then an arg-ile bond), which yields active thrombin and ultimately results in the conversion of fibrinogen to fibrin.
Breakdown of a blood clot or the fibrin network, termed fibrinolysis, requires the conversion of fibrin to a soluble product. This lysis is catalyzed by the proteolytic enzyme plasmin, which circulates in an inactive form, plasminogen. Tissue plasminogen activator (tPA), bacterial hemolytic enzymes (e.g., streptokinase), and proteolytic human enzymes found in urine (e.g., urokinase) all activate plasminogen. These materials are typically used in thrombolytic therapy.
Consequently, the coagulation cascade is a suitable target for diagnosing and treating diseases involving dysregulated blood clotting or the absence of clotting. For example, the diagnosis of hemorrhagic conditions such as hemophilia, where one or more of the thirteen blood clotting factors involved in the coagulation cascade may be defective, can be achieved by a wide variety of coagulation tests. In addition, several tests have been developed to monitor the progress of thrombolytic therapy. Other tests have been developed to signal a prethrombolytic or hypercoagulable state, or monitor the effect of administering protamine to patients during cardiopulmonary, bypass surgery. However, the main value of coagulation tests is in monitoring oral and intravenous anticoagulation therapy. Three of the key diagnostic tests are prothrombin time (PT), activated partial thromboplastin time (aPTT), and activated clotting time (ACT).
PT is the time it takes plasma to clot after the addition of tissue factor (obtained from animals such as rabbits, or recombinant tissue factor, or from brains of autopsy patients). This measures the quality of the extrinsic pathway (as well as the common pathway) of coagulation. The PT is most commonly used to monitor oral anticoagulation therapy. Oral anticoagulants such as Coumadin® suppress the formation of prothrombin. The traditional PT test includes blood being drawn into a tube containing liquid sodium citrate, which acts as an anticoagulant by binding the calcium in a sample. Consequently, the PT test is based on the addition of calcium and tissue thromboplastin to the citrated blood sample, and the time the sample takes to clot is measured.
aPTT is the time taken for a fibrin clot to form. This measures the quality of the intrinsic pathway (as well as the common pathway) of coagulation. The aPTT is most commonly used to monitor intravenous heparin anticoagulation therapy. Heparin administration has the effect of suppressing clot formation. The traditional aPTT test includes blood being drawn into a tube containing liquid sodium citrate, which acts as an anticoagulant by binding the calcium in a sample. Consequently, the aPTT test is based on the addition of activating agent, calcium, and a phospholipid to the citrated blood sample (e.g., a platelet poor plasma), and the time the sample takes to form a fibrin clot is measured.
ACT is the time taken for whole blood to clot upon exposure to an activator. The intrinsic pathway test evaluates the intrinsic and common pathways of coagulation. The ACT is most commonly used to monitor the effect of high-dose heparin before, during, and shortly after procedures that require intense anticoagulant administration, such as cardiac bypass, cardiac angioplasty, thrombolysis, extra-corporeal membrane oxygenation (ECMO) and continuous dialysis. The traditional ACT test includes whole blood being added to a tube containing a surface activator (e.g., celite, kaolin, or glass balls), which results in the activation of the coagulation cascade via the intrinsic (Factor XII) pathway. Consequently, the ACT test is based on the addition of an activator to the intrinsic pathway to fresh whole blood to which no exogenous anticoagulant has been added, and the time the sample takes to form a fibrin clot is measured.
Coagulation monitors are known for the analysis of whole blood. For example, a capillary flow device has been described in U.S. Pat. No. 4,756,884 in which dry reagents are placed into an analyzer, which is then heated to 37° C. before a drop of blood is introduced. The sample is mixed with the reagent by capillary draw. The detection mechanism is based on laser light passing through the sample. Blood cells moving along the flow path yield a speckled pattern specific to unclotted blood. When the blood clots, movement ceases producing a pattern specific to clotted blood. A bibulous matrix with dried coagulation reagents has been devised for a single coagulation test in a device (See, e.g., U.S. Pat. No. 5,344,754) with integrated means for determining a change in electrical resistance upon addition of a sample to the matrix. Detection of the reaction is based on a separate optical assembly that is aligned with and interrogates the bibulous region of the device.
Coagulation point of care assays are also known for the analysis of fluid samples or biological samples. For example, point of care cartridges for conducting a variety of assays responsive to a change in the viscosity of a fluid sample, including assays involving whole blood coagulation, agglutination, fibrinolysis tests and, generally, assays for obtaining information on the clotting or lytic (lysis) process are known (See, for example, U.S. Pat. Nos. 5,447,440 and 5,628,961, which are incorporated herein by reference in their entireties). Additionally, point of care cartridges that provide a means by which a blood sample can be metered and quantitatively mixed with reagents that activate the primary or secondary pathway of the coagulation cascade for subsequent detection of clot formation using a microfabricated sensor are known (See, for example, U.S. Pat. Nos. 6,750,053; 7,923,256; 7,977,106 and 6,438,498, which are incorporated herein by reference in their entireties).
However, coagulation point of care assay systems configured to perform the aforementioned coagulation assays of fluid samples generally comprise the reagent and substrate printed in a dissolvable form on a cover or base of the point of care cartridge or testing device. During analysis, the sample is pushed and pulled by a mechanical process to dissolve and mix the reagent and substrate into the sample. This arrangement of having the reagent and substrate printed in this form in combination with the requirement for mixing the reagent and substrate into the sample has hindered the integration of coagulation tests into a single point of care cartridge or testing device because of the potential for cross-activation of the two distinct coagulation cascade pathways. Accordingly, the need exists for improved point of care cartridge or testing device design that allows for a combination of coagulation tests to be performed on a single point of care cartridge or testing device.